The present invention relates to cleaning exhaust gas of an automobile engine, in particular, to air-fuel ratio control on an automobile equipped with a lean NOx catalyst.
In engines burning fuel at lean side of an air-fuel ratio (hereinafter referred to as a lean-burn engine), a lean NOx catalyst (LNC) is employed to occlude NOx exhausted at lean air-fuel ratio and to reduce the occluded NOx at rich or theoretical air-fuel ratio. Since the amount of NOx that the lean NOx catalyst can occlude is limited, the engine cannot continue the lean-burn operation for a long time. To continue the lean-burn operation for a long time, the air-fuel ratio need to be changed temporarily to rich side to reduce the NOx occluded in the lean NOx catalyst during the lean-burn operation, causing the catalyst to release the occluded NOx. This process is referred to as xe2x80x9cshift-to-rich processxe2x80x9d.
It is well known in the art to perform the shift-to-rich process periodically during the lean-burn operation to actively reduce the NOx occluded in the LNC, and to perform the shift-to-rich process on transition from the lean-burn operation to stoichiometric air-fuel ratio operation (hereinafter referred to as xe2x80x9cstoichiometric operationxe2x80x9d) to reduce the NOx occluded in the LNC. It is also known to estimate the amount of the NOx occluded in the LNC in order to vary the air-fuel rich amount according to the estimated amount, one example of which is disclosed in The Japanese Laid-Open Patent Application No. 7-139340.
According to this application, it is disclosed to provide a NOx estimating counter for estimating the amount of NOx occluded in the LNC. This counter is incremented during the lean-burn operation and is decremented while performing the shift-to-rich process or during the stoichiometric operation. More specifically, a value depending on operation state of the engine is added to the NOx estimating counter at every certain period during the lean-burn operation. A value depending on the fuel amount exceedingly fed to the engine is subtracted from the counter at every certain period while performing the shift-to-rich process or during the stoichiometric operation.
The Japanese Laid-Open Patent Application No. 11-6421 describes other technique which comprises means for determining a saturation state of NOx amount occluded in the NOx catalyst to adequately change the rich amount of the air-fuel ratio, causing the shift-to-rich process to be initiated with appropriate timing. More specifically, this technique includes a calculator for calculating the amount of NOx generated in the engine in accordance with a detected value of internal pressure in cylinder of the engine. When the calculated amount is determined as saturation state of NOx occluded in the NOx catalyst, the shift-to-rich process is initiated. Predetermined period according to engine speed is set in a timer and shift-to-rich process is performed during this set period.
In the above-mentioned conventional technique, however, the period for the shift-to-rich process is set according to the engine speed and therefore the shift-to-rich process is not adapted appropriately to the change of engine load. More specifically, in the shift-to-rich process when the lean NOx catalyst is saturated, sufficient CO and HC have to be supplied to the lean NOx catalyst enough to reduce the occluded NOx. However, setting the period according to only the engine speed, it is probable that the amount of reducing agent (HC, CO) becomes short and can not reduce all of the NOx or the amount of reducing agent becomes too much and may degrade exhaust performance. Moreover, in the case that a three-way catalyst is provided upstream of the lean NOx catalyst, the reducing agent may become short because the reducing agent is oxidized by the three-way catalyst during the shift-to-rich process, as in the case of the air-fuel ratio changing from lean side to stoichiometry.
Considering the above-mentioned problems, it is objective of the invention to provide an electronic control unit for cutting the emission as well as improving the drivability by appropriately controlling the shift-to-rich process for reducing the NOx occluded in the lean NOx catalyst during the shift-to-rich process and transition from the lean operation to the stoichiometric operation.
To solve the above-mentioned problems, according to one aspect of the invention, an electronic control unit is provided which controls an air-fuel ratio of an engine having a lean NOx catalyst in its exhaust system, comprising: an estimator for estimating NOx amount occluded in the lean NOx catalyst; means for performing shift-to-rich process responsive to the NOx amount estimated by said estimator exceeding a predetermined value; a calculator for calculating an accumulated value of an exhaust flow amount; and means for completing said shift-to-rich process responsive to said accumulated value of the exhaust flow amount exceeding a threshold value.
Completing the shift-to-rich process based on the accumulated value of the exhaust flow amount, which is correlated with the amount of the reducing agent (HC, CO), enables the NOx occluded in the lean NOx catalyst to be precisely reduced, and resultingly the exhaust gas can be cleaned at higher level.
According to second aspect of the invention, an electronic control unit is provided for controlling an air-fuel ratio of an engine having a lean NOx catalyst in its exhaust system, comprising: an estimator for estimating NOx amount occluded in the lean NOx catalyst; means for performing shift-to-rich process on a transition from lean-burn operation to stoichiometric operation; a calculator for calculating an accumulated value of an exhaust flow amount; and means for completing said shift-to-rich process responsive to said accumulated value of the exhaust flow amount reaching a threshold value which is set based on the occluded NOx amount estimated by said estimator.
On the transition from the lean-burn operation to the stoichiometric operation, the shift-to-rich process is performed to reduce the lean NOx catalyst and the timing for completing the shift-to-rich process is determined based on the accumulated value of the exhaust flow amount and the estimated amount of the occluded NOx, enabling the NOx occluded in the lean NOx catalyst to be precisely reduced and resultingly the exhaust gas is cleaned at higher level.
According to third aspect of the invention, said electronic control unit further comprises a three-way catalyst and an air-fuel ratio sensor upstream of said lean NOx catalyst, wherein said calculating means starts to accumulate the exhaust flow amount when said air-fuel ratio sensor indicates rich.
Since the accumulation of the exhaust flow amount is started when the air fuel ratio sensor upstream of the lean NOx catalyst indicates rich, the effect of degradation of the three-way catalyst placed upstream of the sensor may be avoided and the NOx occluded in the lean NOx catalyst may be reduced appropriately.
According to fourth aspect of the invention, said electronic control unit further comprises adding means for adding NOx amount to be occluded in said lean NOx catalyst during said shift-to-rich process, to said estimated occluded NOx amount.
It is possible to reduce the NOx occluded in the lean NOx catalyst on the transition from the lean-burn operation to the stoichiometric operation.